1. Technical Field Art
The invention relates to a method and apparatus for reducing noise in signals transmitted via communications channels and is especially, but not exclusively, applicable to the suppression of common mode noise, including radio frequency interference and/or impulse noise, in digital subscriber loops of telecommunications systems. The invention is especially applicable to two-wire or xe2x80x9ctwisted pairxe2x80x9d subscriber loops.
2. Background Art
In the telephone system, noise may comprise radio frequency interference (RI) produced by commercial radio stations in the vicinity of the communications channel. Impulse noise may be caused by a number of phenomena, including switching transients in the central office equipment, or the station apparatus, or from electrical power equipment connected to power lines that run adjacent the telephone subscriber loops. Impulse noise may also be caused by technicians working on the subscriber loops, or even by lightning. Generally, impulse noise will occupy a broader bandwidth than RFI.
When signals transmitted in telephone subscriber loops were at relatively low frequencies, perhaps 3,000 Hz or 4,000 Hz, common mode noise could be dealt with adequately by using twisted wire cable and hybrid transformers to help cancel out any induced interference noise. With the introduction of digital subscriber loops, especially very high speed digital subscriber loops (VDSL) and asymmetric digital subscriber loops (ADSL), the operating frequency approaches the radio frequency bands and conventional techniques, such as balancing of the cable, are no longer sufficient to suppress radio frequency or impulse noise.
Copending Canadian patent application No. 2,237,460, filed May 13, 1998, discloses a method of reducing radio frequency interference in digital subscriber loops in which a common mode signal is extracted from the Tip and Ring of the subscriber loop and applied to a plurality of narrowband filters which are tuned to a corresponding plurality of passbands. A noise detection unit detects the noisiest passband and tunes one of the narrowband filters to that passband. The process is repeated for each of the other narrowband filters in turn to suppress the RFI signals in the corresponding bands. Such adaptive techniques are not suitable, however, for suppressing impulse noise which typically has a very short duration, a relatively wide bandwidth, and occurs substantially randomly so that it has no xe2x80x9chistoryxe2x80x9d allowing adaptation to be used.
U.S. Pat. No. 5,410,264 (Lechleider) issued April 1995 discloses a technique for cancellation of impulse noise in digital subscriber loops. The technique is predicated upon the assumption that, for a particular installation, the impulse noise will generally not be totally random in shape, size and time of occurrence and so can be replicated. Accordingly, Lechleider discloses a technique for estimating one or more of the shape, amplitude and arrival time of an impulse in order to produce a replica which is then subtracted from the original signal. Lechleider is concerned only with impulse noise and his technique cannot be used for radio frequency interference. A further disadvantage is the need for complex calculations to detect impulses and produce replicas.
An object of the present invention is to eliminate, or at least mitigate, some or all of these disadvantages and provide a noise suppression circuit which is better suited to the suppression of radio frequency and/or impulse noise in communications channels.
According to the present invention, there is provided noise suppression apparatus for a communications channel, comprising a hybrid device for coupling to the channel for providing a differential signal corresponding to a signal received from the channel; extraction means for coupling to the channel for extracting a common mode signal from said channel; a noise estimation unit comprising means for deriving from said common mode signal a plurality of digital signals each representing a different one of a plurality of frequency hands of the common mode signal and a plurality of noise detection means for deriving from the digital signals a plurality of noise estimate signals, respectively; means for combining selected samples of the noise estimate signals to provide a common mode noise estimate signals; a delay unit coupled to an output of the hybrid device for delaying said differential signal such common noise estimating signal corresponds temporally with the differential signal; and meaning for subtracting the common mode noise estimate signal from delayed differential to form a noise reduced output signal; wherein each of the noise detector means comprises selector means for selecting a respective one of said selected samples independence upon whether or not a measure of a plurality of previous sample of the corresponding one of the digital signal exceeds a predetermined threshold level.
In preferred embodiments, the noise estimation means comprises an analysis filter bank responsive to the common mode signal for producing a plurality of subband signals, each at a different one of the plurality of different frequencies, a plurality of noise detection means, each coupled to the analysis filter bank to receive a respective one of the plurality of subband signals and provide therefrom a component of said common mode noise estimate signal, and a synthesis filter bank for processing the common mode noise signal components from the plurality of noise detection means to provide said noise suppressed output signal.
The common mode signal extracted from the channel is analog, but maybe converted to a digital signal by an analog-to-digital converter between the extraction means and the digital noise estimator. Each noise detection means may then comprise means operable in each sample period for monitoring and Summing a plurality of previous samples of the corresponding subband signal, means for comparing the sum with a predetermined threshold, and selector means for select in dependence upon said comparison, either a zero value or an instant value of the inverted subband signal and supplying the selected value to a respective one of a plurality of subband inputs of the synthesis filter bank.
The analysis filter bank and the synthesis filter bank may comprise multiresolution filter banks, some of the subband signals having narrower bandwidths than others of the subband signals.
The term xe2x80x9csubband signalsxe2x80x9d is used herein to refer to a plurality of narrowband signals produced by an analysis filter bank of the kind disclosed in an article entitled xe2x80x9cPerfect-channel Splitting By Use of Interpolation and Decimation Tree Decomposition Techniquesxe2x80x9d, Proc. Intl. Conf. Inform. Sci. Syst., pp. 443-446, Aug. 1976, by A. Crosier, D. Esteban and C. Galand. Such analysis filter banks permit xe2x80x9cperfect reconstructionxe2x80x9d of the original signal by means of a complementary synthesis filter bank. For a more recent discussion of the subband transforms involved, which include certain wavelet transforms, the reader is directed to an article entitled xe2x80x9cWavelet and Subband Transforms: Fundamentals and communication Applicationsxe2x80x9d, Ali N. Akansu et al, IEEE Communications Magazine, Vol. 35, No. 12, December 1997. Both of these articles are incorporated herein by reference. Providing the analysis filter bank and synthesis filter bank satisfy certain conditions, as set out in the article by Akansu et al, xe2x80x9cperfect reconstructionxe2x80x9d can be achieved. In a practical implementation, such as in a telecommunications system, some distortion may be acceptable, so it may be possible to use an analysis filter bank which does not quite meet the conditions set out in Akansu et al""s article, and provides only so-called xe2x80x9cpseudo perfect reconstructionxe2x80x9d.
In the context of the present invention, and hereafter in this specification, the term xe2x80x9canalysis filter bankxe2x80x9d refers to a filter bank meeting the afore-mentioned conditions for xe2x80x9cperfect reconstructionxe2x80x9d, or the conditions for xe2x80x9cpseudo-prefect reconstructionxe2x80x9d, and the term xe2x80x9csubband signalsxe2x80x9d refers to signals produced by such an analysis filter bank.